In conventional mass flow meters, heat is applied to a sensing tube conducting the fluid to be measured or is directly applied to a fluid and the temperature of the fluid is measured before and after the addition. When the upstream temperature is equal to the unheated stream temperature, mass flow can be measured as inversely proportional to the temperature difference for a constant heat addition. A conventional bridge circuit can be used to obtain an electrical signal versus flow function. In another arrangement, heat is applied to a sensing tube and the temperature of the tube measured before and after the heat addition. The upstream temperature of the fluid is influenced by the heating of the tube and is nearly equal to the heater temperature at zero flow. The mass flow of the fluid is proportional to the temperature differential for a constant heat addition. In a third arrangement, heat is applied to a very small wire, probe or thermistor in the fluid stream and the cooling effect of the fluid stream is measured. Cooling of the element is a function of the mass flow. In still another arrangement, heat is applied uniformly to a tube by resistance heating and the cooling effect of the fluid measured with thermocouples to determine mass flow. In yet another arrangement, described in detail in my earlier application, referred to above, a pair of temperature sensitive resistance wire coils are wound around the outer surface of a sensing tube through which the fluid flows. The coils are heated and the rate of mass flow of the fluid, which is directly proportional to the temperature differential of the coils, is measured by a bridge circuit.
Known prior art includes U.S. Pat. Nos. 946,886; 1,043,983; 1,193,488; 1,218,717; 1,222,494; 1,254,374; 1,261,086; 1,601,513; 2,594,618; 2,832,018; 2,953,022; 2,972,885; 2,994,222; 3,056,295; 3,181,357; 3,229,522; 3,246,523; 3,251,225; 3,433,068; 3,435,676; 3,500,686; and 3,650,151; German patent 876,484 and British patent 673,143. A publication of interest is "Variable Resistance Sensors Work Better With Constant Current Excitation", by C. A. Bowes, Instrumentation Technology (1966).
The last mentioned arrangement, i.e., using a pair of resistance coils, provides increased sensor efficiency due to reduced loss of heat in the sensor elements. The spacing and size of the sensor elements permits fast response to changes in flow. The sensor elements sense temperatures produced by their own dissipation, thereby keeping thermal lag to an absolute minimum.
While such a dual coil arrangement enables the measurement of mass flow of a fluid over a wide temperature range, satisfactory operation can be obtained substantially only at the attitude at which the sensing tube is oriented when the bridge circuit is balanced. With the sensing tube located even in a still air environment, convective air currents around the coils carry the generated heat up and away. As the sensing tube is tilted so that one coil is higher than the other, heat generated from the lower coil is carried to the higher coil and raises its resistance which, therefore, unbalances the bridge circuit. The bridge circuit is balanced at no flow and with the sensing tube horizontal (or with both coils at the same level if the tube is bent). Therefore, the bridge circuit output is influenced by the position or attitude of the sensing tube and is not simply a function of the flow rate of the fluid.
The present invention provides reduced flow meter sensitivity to changes in attitude and is accomplished by encasing the sensing tube coils in an open cell foam material. The result is a minimization of natural convection around the sensor coils and a reduction in attitude sensitivity from greater than 50%, without the encasement, to less than 0.5%.
The advantages of this invention, both as to its construction and mode of operation, will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures.